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Title: A New Concept of Applying Methanol to Dry Cellulose Insulation at the Stage of Manufacturing a Transformer
Author: Piotr Przybylek

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energies
Article

A New Concept of Applying Methanol to Dry
Cellulose Insulation at the Stage of Manufacturing
a Transformer
Piotr Przybylek

ID

Institute of Electrical Power Engineering, Poznan University of Technology, Piotrowo 3A,
60-965 Poznan, Poland; piotr.przybylek@put.poznan.pl; Tel.: +48-061-665-2018
Received: 13 June 2018; Accepted: 25 June 2018; Published: 26 June 2018




Abstract: A decisive technical challenge for transformer manufacturers is correctly drying
the cellulose insulation. During the production of a transformer, it is necessary to reduce its
insulation’s moisture content from about 8% to less than 1% in the shortest possible time period.
The drying of insulation is a time-consuming process, and for high-power transformers, it can last
up to three weeks. Several drying techniques are used during the production of a transformer,
and all of them require heating up the insulation to a high temperature and applying a vacuum.
Unfortunately, the use of a high drying temperature above 100 ◦ C can cause a decrease in the degree
of cellulose polymerization by over a dozen percentage points. This paper presents a new concept
for drying cellulose insulation that does not require heating insulation and applying a vacuum.
In this solution, methanol is used as the drying medium. The research results showed the possibility
of drying cellulose insulation by means of methanol with different initial moisture contents.
The possibility of completely drying pressboard of various thicknesses for a sufficient period of
time was also proven. The paper also presents a new concept of both the device and the procedure
for drying cellulose insulation by means of methanol.
Keywords: transformer; cellulose insulation; drying; methanol

1. Introduction
Power transformers play a significant role in the power system, both at the stage of electric
energy generation (generator set-up transformers) and at the stage of its transmission and distribution
(grid and distribution transformers). The reliability of transformers is a prerequisite for ensuring a
continuity of electricity supply. Due to the high price of transformers, the maximum working time
of these devices is extended, and in some cases, the lifetime of power transformers exceeds even
50 years [1,2].
The insulating system is a critical element of every transformer [3]. The most common solution is
an insulating system made of cellulose materials (paper, pressboard) impregnated with mineral oil [4].
This is a proven solution that has been used on a large scale for nearly 100 years [5]. Unfortunately,
during long-time exploitation, the insulation system undergoes aging caused by oxidation, thermolysis,
and hydrolysis processes. The presence of moisture in solid and liquid transformer insulation plays
a critical role in the transformer’s life [6]. Moisture has been recognized as “enemy number one” of
transformer insulation [7].
Water is both a catalyst in the process of cellulose depolymerization and the product of its
oxidation. The water content in a transformer insulation system changes during its operation.
The rate of the moistening process depends both on the design of the transformer and its load [8,9].
For transformers with a membrane-sealed conservator preservation system, the rate of water

Energies 2018, 11, 1658; doi:10.3390/en11071658

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Energies 2018, 11, 1658

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contamination is about 0.03% to 0.06%, while for transformers with an open-breathing conservator,
it is up to 0.2% per year [8].
The high level of water content in transformer insulation is dangerous not only due to
the aging process of cellulose [10–12], but also due to a decrease in the insulation system’s electrical
strength [13–16], an increase in the probability of the appearance of partial discharges [17–19],
and evolution of water vapor bubbles [20,21]. The presence of water in the cellulose insulation
improves its thermal conductivity [22,23] but unfortunately significantly deteriorates the other
above-mentioned parameters.
Thus, it is very important that a transformer, after the manufacturing process, has insulation with
the lowest possible moisture content. The conditions prevailing in the transformer factory (air humidity
and temperature) may cause the water content in the cellulose insulation to reach even eight percentage
points by weight. Such insulation has poor dielectric properties; therefore, it must be dried before
being impregnated with oil. Reducing the moisture level constitutes a challenge for power transformer
manufacturers who want to offer high-quality products with acceptable residual moisture.
2. Drying Methods of Transformer Insulation
One of the requirements of a potential transformer’s customer is a low level of the moisture
insulation. The moisture of cellulosic materials after the production process should not exceed 1%,
but the water content in very well dried insulation is even lower than 0.5% [8,24]. This is the average
value of moisture of only a part of the cellulose insulation, which is determined most often by methods
based on dielectric spectroscopy, such as FDS—frequency-domain spectroscopy, PDC—polarization
and depolarization measurement, or RVM—recovery voltage measurement. It should be noted that the
transformer’s insulation system is very complex, and consists of cellulose materials of various thickness
and density. This determines both the time and the parameters of the drying process [25,26]. After the
drying process, both spaces with very well dried (<0.5%) and with poorly dried cellulose (>1%) can
be found in the insulation system. Material that undergoes the drying process very well is thin,
winding paper whose total thickness usually does not exceed 1 mm. In turn, the drying of thick
elements such as angle rings, cylinders, or spacer blocks presents many problems.
Both the time and final effect of drying depend strongly on the method that is used. The insulation
drying methods that are used at the stage of transformer manufacturing require the heating of cellulose
and the use of a suitable underpressure. The heating up of the insulation system is achieved by:





Placing the insulation system in a vacuum dryer—conventional method
Heat of solvent evaporation—vapor phase drying method
Direct or low frequency current flow through the windings—LFH method (low-frequency heating)

The conventional drying method consists of heating the insulation system in the dryer by using
hot air and reducing the pressure. The insulation system is heated to a temperature of 85–130 ◦ C.
After heating, the pressure in the dryer is lowered to below 1 mbar, which results in an increase
in the rate of water evaporation [27]. Unfortunately, the evaporation of water is accompanied by a
decrease in the temperature of the insulation. The reheat of insulation by heaters placed in the walls
of the dryer is ineffective due to the small temperature difference between the walls of the dryer
and the insulation system [28]. For this reason, several cycles of insulation heating and pressure
reduction are needed in order to dry the insulation sufficiently. This significantly extends the drying
time of the insulation.
The vapor-phase drying method consists of heating the insulation system with the heat of solvent
vapor condensation and reducing the pressure [28–30]. Solvent vapor at a high temperature of about
130 ◦ C is introduced into the dryer, in which the pressure is reduced to 7 hPa. This way, the effect of
immediate condensation of the solvent on the surface of the insulation system is obtained, which allows
for fast heating of the insulation [28]. Solvent vapors reach hard-to-reach areas of the insulation
system, which enables its uniform heating. Another advantage of this method is the washing out of

Energies 2018, 11, 1658

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post-production impurities from the insulation system. Solvent losses are estimated at about 1–1.5% of
the mass of dried insulation [28]. The remaining residual solvent in the cellulose insulation is dissolved
by the oil. One of the significant disadvantages of this method is the risk of explosion. Solvent vapor
with air is an explosive mixture; therefore, extreme caution is required during the drying process.
Another method used to dry the transformer consists of heating the insulation as a result of a
direct current or low-frequency current flowing through the windings and applying a vacuum. A more
technically advanced and more efficient drying technique is the use of the low-frequency heating
(LFH) method. In this method, the insulation is heated from the inside with a low frequency current
supplied to the high-voltage (HV) windings. The low-voltage (LV) windings remain short circuited.
Usually, a current frequency in the range of 0.4 Hz to 2 Hz is used [31]. The reduced frequency
enables obtaining a suitable value of the heating current at a relatively low supply voltage. The LV
windings are heated by the current flowing as a result of the transformation of the voltage from
the HV windings. HV and LV windings can heat up to typical drying temperatures of 110–120 ◦ C [31].
During electrical heating, the vacuum level is kept at approximately 30 mbar for safety reasons
(Paschen’s law). After heating is stopped, the pressure is lowered to below 1 mbar [32]. Drying by
means of the LFH method can be assisted by spraying the windings with hot oil, which is done to
improve the drying dynamics and heating uniformity of the whole of the insulation. It should be
noted that in this method, the insulation of the windings (paper wrapped around a copper wire, radial
spacers) is mainly heated, and the elements that are distanced from the windings (cylinders or angle
rings) are heated to a much worse degree. For this reason, these elements are much more difficult
to dry.
Heating the insulation to a high enough temperature is of key importance for effective transformer
insulation drying in all of the above-mentioned methods. On one hand, the high temperature improves
the drying process, but on the other hand, it contributes to the degradation of cellulose. The main
component of cellulose insulation is cellulose fibers. These fibers are made of macrofibers, which in
turn are made of microfibers. Microfibers consist of elementary fibers, which are made of cellulose
chains [33]. The cellulose chains consist of β-D-glucopyranosyl units. The number of such units
per chain is called the degree of polymerization (DP). New cellulose paper will have a DP of about
1200–1300 [34,35]. The literature data [11,34] state that the paper in a transformer will have a DP of
about 1000 after the factory drying process.
Table 1 presents the research results of Przybylek [36], which confirm the decrease in the degree
of cellulose polymerization during the factory drying of transformer insulation. The research tests
were carried out for five different cellulose materials that were used as a paper wrap around a copper
wire in power transformers. It was found that, as a result of the drying process, the average decrease
in the degree of cellulose polymerization was about 13.7%.
Table 1. Degree of polymerization (DP) of cellulose before and after factory drying [36].
Material

Paper No. 1

Paper No. 2

Paper No. 3

Paper No. 4

Paper No. 5

DP before drying process
DP after drying process
Decrease of DP during drying
Percentage decrease of DP, in %

1185
1080
105
9

1180
1080
100
8

1238
1059
179
14

1422
1267
155
11

1325
984
341
26

During the power transformer’s operation, the degree of cellulose polymerization gradually
decreases [10,24]. The dynamic of this process depends mainly on the temperature and water content
in cellulose insulation [11,37]. IEC 60076-7 [38] suggests, in accordance with Montsinger’s law, that the
aging of transformer insulation is double (or halved) for every 6 ◦ C in the range of 80–140 ◦ C [11].
A cellulose material with a degree of polymerization below 350 is considered to be aged [39], which is
associated with poor mechanical strength. The exploitation of devices insulated by aged cellulose

Energies 2018, 11, 1658

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materials is risky, particularly in the case of high mechanical stresses, which often lead to winding
movement and damage to fragile insulation [11].
It can be estimated that the decrease in the degree of cellulose polymerization caused by
the process of drying insulation at the stage of transformer manufacturing can result in a shortening of
its technical life by up to five years. This increases the risk of serious failures and also forces faster
transformer repair or replacement.
The temperature of the drying process should be lowered in order to prevent excessive aging
of the insulation at the stage of transformer manufacturing. Unfortunately, the consequence of this
is prolonged drying time and problems with achieving the assumed moisture level, particularly in
thick materials. During transformer operation, this water successively migrates to the oil, and then to
cellulose with a lower relative humidity.
A method that uses methanol as a medium for removing water from cellulose is free of
the disadvantages associated with the drying of the insulation using the techniques described above.
The author put forward the hypothesis that the methanol can be used for drying all of the insulation at
the stage of transformer manufacturing, before impregnating the cellulose with an insulating liquid.
The possibility of using this method for the effective drying of cellulosic materials is demonstrated in
the next chapter.
3. Drying of Cellulose Materials by Means of Methanol
3.1. The Application of Methanol for the Extraction of Water from Fibrous Material—Previous Experience
Very high solubility of water in methanol and its ability to extract water from cellulose was used in
two methods of moisture measurement. The first of these is the standardized Karl Fischer method [40],
while the second is a technique based on the use of near-infrared spectrophotometry [41]. Both methods
have a common feature; namely, they measure the water content in cellulose material and they
require its earlier, total extraction from the tested sample. Such extraction is possible using methanol,
as evidenced by the results of research presented in the CIGRE brochure [42] and publication [41].
The CIGRE brochure [42] presents the results of interlaboratory tests on the water content in
pressboard samples obtained by means of the Karl Fischer titration method. As mentioned above,
for the Karl Fischer reaction, it is necessary to extract water from the sample. For this purpose,
seven laboratories used the evaporative technique for water extraction, while five laboratories used
water extraction with methanol. Similar results were obtained for both techniques of measuring
the water content, which indicate the possibility of using methanol for total water extraction.
In [41] Przybylek described the method of water content measurement in electroinsulating fibrous
materials using near-infrared spectrophotometry (NIR method). This method is based on the extraction
of water from cellulosic material to methanol. In [41], very good agreement was found in the results
obtained by means of the NIR method, the Karl Fischer titration method, and the weight method
for both cellulose and aramid samples that were both non-impregnated and impregnated with mineral
oil. The obtained results revealed the possibility of using methanol for the effective extraction of water
from fibrous materials.
The research results described above constituted a starting point for the tests conducted here to
answer the following question: is it possible to apply methanol for effective drying of a large-sized
transformer’s insulation system at the stage of its manufacturing before oil impregnation? In order
to answer this question, it was necessary to conduct research related to assessing both the influence
of water concentration in methanol and the thickness of cellulosic materials on the effectiveness
and dynamics of the drying process.
3.2. The Influence of Water Concentration in Methanol on the Effectiveness of Cellulose Insulation Drying
The moisture level of cellulose insulation after the transformer manufacturing process results from
the climatic conditions that prevail in the production hall. For an air temperature of 20 ◦ C and a relative

Energies 2018, 11, 1658

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humidity of about 50–60%, the water content in cellulose is about 7–8% [43]. Drying can be considered
effective if the water content decreases to about 0.7 percentage points by weight. This is the level of
moisture that can be obtained by transformer manufacturers using traditional drying techniques, and is
acceptable to their purchasers. The mass ratio of mineral oil to the cellulose materials in the transformer
lies in the wide range of 6:1 to 30:1. The drying procedure assumes filling the transformer tank with
methanol. Taking into account the oil density (0.88 kg/L) and methanol density (0.792 kg/L), the mass
ratio of methanol to cellulose was calculated to be in the range of from 5.4:1 to 27:1. Assuming a
reduction of the water content in cellulose during the drying process by seven percentage points,
the water concentration in methanol, for the mass ratio as calculated above, will be in the range of
about 0.26% to 1.3%.
The effectiveness of water extraction from cellulose by means of methanol has been confirmed in
papers [41,42] and regarding the moisture measurement methods (Section 3.1). However, it should
be noted that a low water concentration in methanol is recommended in both methods of measuring
the water content. According to the standard [40], the water content in methanol intended for water
extraction from cellulose should not exceed 0.02%, while the water content in methanol after this
process should be lower than 0.126%. The question then arises of whether it is possible to effectively
dry the transformer’s cellulose insulation with methanol in which the water concentration is very high
and reaches a level of about 1%. To answer this question, an experiment was designed to evaluate
the effectiveness of cellulose paper drying by means of methanol at varying water concentrations.
3.2.1. Measurement Procedure
Methanol that was “pure for analysis” was used for the research. According to the manufacturer,
its initial water concentration was about 0.02%. This methanol was poured into four vials that were
each 45 mL in volume. Subsequently, various volumes of water were added to the three vials so
as to obtain methanol samples with a moisture content of about 0.2%, 0.6%, and 1%. This moisture
corresponded to the different drying conditions of the transformer insulation system. The initial
concentration of water (Cpi ) in the methanol samples was measured using the Karl Fischer titration
method, according to the standard [40]. The results of these measurements are given in Table 2.
Methanol prepared in this manner was used to dry the cellulose paper samples, which had previously
been conditioned in air at a temperature of 22 ◦ C and a humidity of 55%. The thickness of the paper
was 0.055 mm, and its grammage was 47.6 g/m2 .
An evaluation of the dynamics and efficiency of drying cellulose paper by means of methanol
with different concentrations of water was possible due to the use of near-infrared spectrophotometry.
The near-infrared spectrophotometry method was used to measure the water content in fibrous
materials and is described in detail by Przybylek in [41]. In this method, the absorbance of a wave
that was 1939 nm in length passing through methanol is measured. In methanol, water is extracted
beforehand from the cellulose. From Formula (1) taken from [41], the water concentration in methanol
resulting from its extraction from cellulose material is calculated. On the basis of this concentration
and the mass of methanol (Mm ) in the spectrophotometer cuvette, the mass of water in methanol (Mw )
is determined. Knowing both the mass of water and mass of paper (Mp ), it is possible to calculate
the percentage of water content in cellulose (WCPNIR ).
A Jasco V-570 spectrophotometer and a cuvette with an optical path of 10 mm made of Infrasil
quartz glass were used to measure the absorbance. The transmittance of Infrasil quartz glass is about
95% for the chosen wavelength. The cuvette selected for the experiments was additionally equipped
with a screw cap with a silicon membrane covered with polytetrafluoroethylene, which allowed
eliminating the problem of methanol evaporation during the measurements.

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The procedure that was used for assessment of the effectiveness and dynamics of the cellulose drying
process by means of using methanol at varying water concentrations included the following steps:

2.

Filling the spectrophotometer cuvette with methanol of mass (Mm ) and water concentration (Cpi )
measured by means of the Karl Fischer Titration (KFT) method
Measurement of absorbance (Absi ) for methanol at water concentration (Cpi )

3.
4.
5.
6.

Inserting a paper sample of mass (Mp ) into a cuvette filled with methanol
Measurement of absorbance (Abst ) after time t
Mixing methanol in the cuvette for 30 s
Repeating steps (4) and (5) every four minutes until the absorbance value (Absf ) is steady

1.

On the basis of the results obtained during realization of the above-described procedure,
the following parameters were calculated: the concentration of water in methanol (Cp ), the mass
of water (Mw ), the water content in paper samples (WCPNIR ), and the concentration of water in
methanol (Cpf ).
Equation (1) taken from Przybylek [41] was used to calculate the concentration of water in
methanol (Cp ) extracted from the cellulose sample:
Cp =

Abs f − Absi − 0.0016
,
0.5237

(1)

The mass of water (Mw ) extracted from the paper sample was calculated from the formula:
Mw =

C p · Mm
,
100 − C p

(2)

To calculate the water content in paper samples (WCPNIR ), the following formula was used:
WCPN IR =

Mw
× 100,
M p − Mw

(3)

The concentration of water in methanol (Cpf ) after drying the paper samples was calculated from
the formula:
C p f = C pi + C p .
(4)
3.2.2. Research Results and Discussion
The research results obtained in the course of the above-described procedure are presented in
Table 2.
Table 2. Results of measurements used to calculate the water content in paper (WCPNIR ) after the drying
process—evaluation of the efficiency of water extraction from cellulose paper by means of methanol at
various water concentrations.
Sample

Cpi
%

Mm
g

Absi
-

Mp
g

Absf
-

Cp
%

Mw
mg

WCPNIR
%

Cpf
%

0.02%
0.2%
0.6%
1%

0.0236
0.2102
0.5961
1.0121

3.7713
3.7550
3.8197
3.8156

0.9773
1.0801
1.2881
1.5127

0.0771
0.0739
0.0745
0.0737

1.0568
1.1541
1.3596
1.5864

0.1487
0.1384
0.1335
0.1376

5.61
5.19
5.10
5.25

7.81
7.54
7.33
7.62

0.1723
0.3485
0.7296
1.1497

The water concentration in methanol Cpt and the water content in paper WCPNIRt were calculated
based on data from Table 2, absorbance Abst measured after time t from the start of water extraction,
and the transformed Formulas (1)–(4). The results of these calculations are shown in Figure 1.

Energies 2018, 11, 1658

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An analysis of the results that is presented in Table 2 and Figure 1 showed that the paper drying
efficiency was similar for methanol, with an initial moisture content in the range of 0.02% to 1%.
The drying dynamics were the same in all of the cases. The highest rate of drying was observed during
the first 20 min of the experiment. During this time, the water content in the paper samples decreased
from about 7.6% to about 0.7%. In all of the cases, the paper-drying process ended after 39 min from
the beginning of water extraction. After this time, a fluctuation of the water concentration in methanol
was observed, which was related to absorbance resolution.
The research results show the possibility of using methanol, even with a significant concentration
of water
exceeding
1%,PEER
for REVIEW
the efficient drying of a transformer’s cellulose insulation.
Energies
2018, 11, x FOR
7 of 13

Figure 1. Comparison of water concentration in methanol Cpt and water content in paper WCPNIRt

Figure 1. Comparison of water concentration in methanol Cpt and water content in paper WCPNIRt
depending on drying time for initial water concentration in methanol 0.02%, 0.2%, 0.6%, and 1%.
depending on drying time for initial water concentration in methanol 0.02%, 0.2%, 0.6%, and 1%.

3.3. The Influence of Water Concentration in Methanol on the Effectiveness of Cellulose Insulation Drying

3.3. The Influence of Water Concentration in Methanol on the Effectiveness of Cellulose Insulation Drying
The thickness of cellulose materials has a very large impact on the rate and efficiency of drying

The
thicknessbyofusing
cellulose
materials
has
a very large
impact
on methanol
the rate and
efficiency
of drying
the insulation
all of the
methods,
including
the one
based on
as a drying
medium.
the insulation
by
using
all
of
the
methods,
including
the
one
based
on
methanol
as
a
drying
A drying time of the transformer cellulose insulation that is too long significantly increases the medium.
cost
A drying
time of the of
transformer
insulation
that is that
too long
increases
thethe
cost of
of manufacture
the device.cellulose
In turn, drying
efficiency
is toosignificantly
low does not
allow for
elimination
of device.
water from
thickdrying
elements
of the that
insulation
system.
thefor
transformer’s
manufacture
of the
In turn,
efficiency
is too low
doesDuring
not allow
the elimination
operation,
waterelements
from thick
migrates
slowlyDuring
to otherthe
parts
of the system,
and is a catalyst
of water
from thick
ofstructures
the insulation
system.
transformer’s
operation,
water from
in
the
aging
process
of
both
solid
and
liquid
insulation.
thick structures migrates slowly to other parts of the system, and is a catalyst in the aging process of
The aim of the research conducted here was to check the effectiveness and dynamics of drying
both solid and liquid insulation.
cellulose materials of various thicknesses by means of methanol. Table 3 presents the properties of
The aim of the research conducted here was to check the effectiveness and dynamics of drying
the materials selected for the research. Before drying, these materials were conditioned in air at 23 °C
cellulose
of 30%.
various thicknesses by means of methanol. Table 3 presents the properties of
and amaterials
humidity of
the materials selected for the research. Before drying, these materials were conditioned in air at 23 ◦ C
and a humidity of 30%. Table 3. The properties of the investigated cellulose materials.
Thickness Basic Weight Density
Sample
Table 3.
The properties of the investigated cellulose
materials.
mm
g/m2
kg/m3
Pressboard 0.5 mm
0.548
515
939
Sample
Thickness mm
Basic Weight g/m2
Density kg/m3
Pressboard 1 mm
1.110
1167
1051
Pressboard 0.5
mm
515
939
Pressboard
2 mm0.548 2.104
2354
1119
Pressboard Pressboard
1 mm
1.110
1167
1051
3 mm
3.384
3908
1155
Pressboard 2 mm
2.104
2354
1119
PressboardProcedure
3 mm
3.384
3908
1155
3.3.1. Measurement
The procedure described in Section 3.2 was applied in order to evaluate the drying efficiency of
pressboard of different thicknesses, with the exception that subsequent absorbance measurements
wereprocedure
taken everydescribed
3 min. Thein
mass
ratio of
to cellulose
material
was 8:1,the
which
corresponds
The
Section
3.2methanol
was applied
in order
to evaluate
drying
efficiency of
to
the
mass
ratio
of
oil
to
cellulose
that
can
occur
in
a
power
transformer.
pressboard of different thicknesses, with the exception that subsequent absorbance measurements

3.3.1. Measurement Procedure

Energies 2018, 11, 1658

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were taken every 3 min. The mass ratio of methanol to cellulose material was 8:1, which corresponds
to the mass ratio of oil to cellulose that can occur in a power transformer.
3.3.2. Research Results and Discussion
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8 of 13

Table 4 shows the research results obtained in accordance with the procedure described above.
3.3.2. contains
Research Results
and of
Discussion
The table
the results
water content measurement in paper (WCPNIR ) obtained by means of
the spectrophotometric
method.
Table 4 shows the
research results obtained in accordance with the procedure described above.
The table contains the results of water content measurement in paper (WCPNIR) obtained by means of
Table
4. Results of measurements
the
spectrophotometric
method. used to calculate the water content in paper (WCPNIR ) after the drying
process—evaluation of the efficiency of water extraction from pressboard of various thickness by means
Table 4. Results of measurements used to calculate the water content in paper (WCPNIR) after the
of methanol.
drying process—evaluation of the efficiency of water extraction from pressboard of various thickness
by means of methanol.
C
Abs
Mm
Abs
Mp
Cp
Mw
WCP

Sample

pi

NIR

f

i

% Cpi
g Mm
-Absi
Sample
Pressboard 0.5 mm 0.0236% 3.7397
g 0.9799
Pressboard
1
mm
0.0236
3.6968
Pressboard 0.5 mm 0.0236 3.7397 0.9799
0.9799
Pressboard
2 mm
Pressboard
1 mm 0.0236
0.02363.7864
3.6968 0.9776
0.9799
Pressboard 3 mm
0.0236 3.6926 0.9776
Pressboard 2 mm 0.0236 3.7864 0.9776
Pressboard 3 mm 0.0236 3.6926 0.9776

gp
M
0.467
g
0,460
0.467
0.469
0,460
0.466
0.469
0.466

-f
Abs
1.3148
1.3242
1.3148
1.3204
1.3242
1.3181
1.3204
1.3181

C%
p
0.6363
%
0.6543
0.6363
0.6516
0.6543
0.6470
0.6516
0.6470

%
Mwmg WCPNIR
23.95 % 5.41
mg
24.35 5.415.59
23.95
24.83 5.595.59
24.35
24.05
24.83
5.595.44
24.05
5.44

Cpf
%

Cpf
% 0.6599
0.6779
0.6599
0.6752
0.6779
0.6706
0.6752
0.6706

The results of water concentration in methanol (Cp ), the mass of water extracted from the sample
results
of water
concentration
in methanol
(Cp), the mass of water extracted from the
(Mw ), theThe
water
content
in pressboard
(WCP
NIR ), and the concentration of water in methanol after
sample (Mw), the water content in pressboard (WCPNIR), and the concentration of water in methanol
the drying process (Cpf ) were calculated by means of Equations (1)–(4), respectively.
after the drying process (Cpf) were calculated by means of Equations (1)–(4), respectively.
The water concentration in methanol Cpt and water content in paper WCPNIRt were calculated
The water concentration in methanol Cpt and water content in paper WCPNIRt were calculated
basedbased
on: data
fromfrom
TableTable
4, absorbance
AbstAbs
measured
after time t from the start of the drying process,
on: data
4, absorbance
t measured after time t from the start of the drying
and transformed
Formulas
(1)–(4).
The
results
of
these
calculations
are shown
in Figure
2. 2.
process, and transformed Formulas (1)–(4). The results of
these calculations
are shown
in Figure

Figure 2. Comparison of water concentration in methanol Cpt and water content in paper WCPNIRt

Figure 2. Comparison of water concentration in methanol Cpt and water content in paper WCPNIRt
depending on the drying time for pressboard with thicknesses of 0.5 mm, 1 mm, 2 mm, and 3 mm.
depending on the drying time for pressboard with thicknesses of 0.5 mm, 1 mm, 2 mm, and 3 mm.

The research results show that the water content in the pressboard samples before drying was
The research results show that the water content in the pressboard samples before drying was
similar and equal to about 5.5%. The drying effect was similar regardless of the material’s thickness.
similar
and
equal
to about
5.5%.
The
drying
effectmaterials
was similar
regardless
of thestrongly
material’s
thickness.
It has
been
shown
that the
drying
time
of cellulose
by means
of methanol
depends
It hason
been
shown
that
the
drying
time
of
cellulose
materials
by
means
of
methanol
strongly
depends
their thickness. The highest dynamics of drying was observed for pressboard samples of the on
theirsmallest
thickness.
The highest
drying was
for pressboard
smallest
thickness.
Figuredynamics
3 shows aof
comparison
of observed
the time necessary
for the samples
reductionofofthe
water
thickness.
3 showsofadifferent
comparison
of the during
time necessary
the reduction
of water
content in
contentFigure
in pressboard
thicknesses
the dryingfor
process
from the initial
moisture
contentof
of different
5.5% to a level
of 1%, 0.5%,
andthe
0.1%
pressboard
thicknesses
during
drying process from the initial moisture content of 5.5%

to a level of 1%, 0.5%, and 0.1%

Energies
Energies 2018,
2018, 11,
11, 1658
x FOR PEER REVIEW
Energies 2018, 11, x FOR PEER REVIEW

9 of 13
9 of 13

Figure 3. Time necessary for reduction of water content in pressboard of different thicknesses during

Figure 3. Time necessary for reduction of water content in pressboard of different
different thicknesses during
the drying process from the initial moisture content of 5.5% to a level of 1%, 0.5%, and 0.1%.
the drying
process
from
the
initial
moisture
content
of
5.5%
to
a
level
of
1%,
drying process from the initial moisture content of 5.5% to a level of 1%,0.5%,
0.5%,and
and0.1%.
0.1%.

For a sample with a thickness of 0.548 mm, the time needed to dry it from 5.5% to 1%, 0.5%, and
For
sample with
with athickness
thicknessofof0.548
0.548mm,
mm,
time
needed
to dryfrom
it from
5.5%1%,
to 1%,
For aawas
sample
thethe
time
needed
to dry
5.5%
0.5%,0.5%,
and
0.1%
equal to 18 amin,
25 min, and 54 min,
respectively,
while the
timeitneeded
to dryto
the sample
and
0.1%
was
equal
to
18
min,
25
min,
and
54
min,
respectively,
while
the
time
needed
to
dry
the
sample
0.1%with
wasaequal
to 18ofmin,
min,
andsame
54 min,
while
thetotime
needed
dry
the335
sample
thickness
3.38425
mm
to the
levelsrespectively,
of moisture was
equal
140 min,
209 to
min,
and
with
aathickness
ofof3.384
mm
toto
the
same
levels
of moisture
waswas
equal
140
min,
209
min,
and
335
The obtained
results
showed
the
possibility
of moisture
using methanol
fortothe
and
effective
drying
withmin.
thickness
3.384
mm
the
same
levels
of
equal
tofast
140
min,
209 min,
andmin.
335
The
obtained
results
showed
the
possibility
of
using
methanol
for
the
fast
and
effective
drying
of
of
cellulosic
materials,
even
those
of
considerable
thickness.
min. The obtained results showed the possibility of using methanol for the fast and effective drying

cellulosic
materials,
even
those
of considerable
thickness.
of cellulosic
materials,
even
those
of considerable
thickness.

4. The Concept of a System for Drying Transformer Cellulose Insulation by Means of Methanol

4.
of aa System
for
Drying
Transformer
Cellulose
Insulation
by
Means
of
Based onofthe
results offor
theDrying
researchTransformer
presented in Section
3, the
concept of by
a device
enabling
the
4. The
The Concept
Concept
System
Cellulose
Insulation
Means
ofMethanol
Methanol
drying
of
solid
transformer
insulation
by presented
means of methanol
was3,
prepared.
The proposed
solution
Based
on
the
results
of
the
research
in
Section
the
concept
of
a
device
enabling
Based onthe
thedrying
resultsofofa the
research at
presented
in Section
3, the concept
of impregnating
a device enabling
the
concerns
transformer
the
stageofof
its manufacture,
before The
the
drying
of solid
transformer
insulation
by
means
methanol
wasprepared.
prepared.
proposed the
solution
drying
of
solid
transformer
insulation
by
means
of
methanol
was
The
proposed
solution
cellulose with an insulating liquid. The device scheme is shown in Figure 4.
concerns
a transformer
at the
of its manufacture,
before impregnating
the cellulose
concerns the
thedrying
dryingof of
a transformer
at stage
the stage
of its manufacture,
before impregnating
the
with
an
insulating
liquid.
The
device
scheme
is
shown
in
Figure
4.
cellulose with an insulating liquid. The device scheme is shown in Figure 4.

Figure 4. System for drying cellulose insulation by means of methanol; (1) insulation system, (2) tank
for methanol with a low water concentration, (3) tank for methanol with an increased level of water,
(4) pump, (5) transformer tank, (6) particulate filter, (7) NIR sensor, (8) three-way valve, (9) pump,
(10) air compressor or nitrogen accumulator, (11) molecular sieve filter, (12) sensor for control of the
concentration
of methanol
vapor, (13)
methanol by
vapor
condenser,
(14) gas (1)
outlet
[36].
Figure
4. System for
drying cellulose
insulation
means
of methanol;
insulation
system, (2) tank

Figure 4. System for drying cellulose insulation by means of methanol; (1) insulation system, (2) tank
for methanol
methanol with
with a low water
water concentration, (3)
for methanol
with an
level of
for
(3) tank
tank
an increased
increased
of water,
water,
The device fora low
drying theconcentration,
insulation system
(1) for
viamethanol
extractingwith
water
by means level
of methanol
(4)
pump,
(5)
transformer
tank,
(6)
particulate
filter,
(7)
NIR
sensor,
(8)
three-way
valve,
(9)
pump,
(4)
pump,
(5)
transformer
tank,
(6)
particulate
filter,
(7)
NIR
sensor,
(8)
three-way
valve,
(9)
pump,
consists of the following elements: two tanks, one used for methanol with a low water concentration
(10) air
air compressor
or nitrogen
accumulator,
(11)
molecular
sieve filter,
(12)
sensor
for for
control
of the
(10)
compressor
nitrogenwith
accumulator,
(11)
molecular
(12)
control
of
(2),
and the
other for or
methanol
an increased
level
of watersieve
(3) asfilter,
a result
ofsensor
the drying
process.
concentration
of methanol
vapor,
(13)(13)
methanol
vapor
condenser,
(14)(14)
gasgas
outlet
[36].
the
concentration
of methanol
vapor,
methanol
vapor
condenser,
outlet
[36].

The device for drying the insulation system (1) via extracting water by means of methanol
consists of the following elements: two tanks, one used for methanol with a low water concentration
(2), and the other for methanol with an increased level of water (3) as a result of the drying process.

Energies 2018, 11, 1658

10 of 13

The device for drying the insulation system (1) via extracting water by means of methanol consists
of the following elements: two tanks, one used for methanol with a low water concentration (2),
and the other for methanol with an increased level of water (3) as a result of the drying process.
By means of a pump (4), a continuous flow of methanol is forced into the closed-cycle drying
transformer (5) and tank (2). A particulate filter (6) is installed in the circulation, which eliminates
the impurities remaining in the insulation system after the transformer manufacturing process.
During the methanol cycle, the water concentration is monitored by means of an NIR sensor (7)
measuring the absorbance of a wave with a length of 1939 nm passing through the methanol containing
the extracted water. The moisture level of the insulation is estimated on the basis of the dependence
of absorbance on the water concentration in methanol and by taking the mass of the dried insulation
into account [41]. After reaching the assumed level of moisture, the three-way valve (8) is set to
pumping the methanol, using the pump (9) to the tank (3). Thus, the prepared insulation system is
blown with dry gas (10—air compressor or nitrogen accumulator) and additionally dried by means of
a molecular sieve filter (11). The concentration of methanol vapor in the gas leaving the transformer
tank is controlled by means of the sensor (12). The methanol vapor is condensed using a condenser
(13) and the methanol runs down into the tank (3), while the gas escapes into the atmosphere via
the outlet (14). Once the methanol is completely evaporated from the insulation, the drying process
is terminated. It is possible to use an additional filter with molecular sieves 3A in a closed cycle
to improve the efficiency of insulation drying. This material enables the effective removal of water
from methanol.
5. Conclusions
The article presents a new concept of using methanol for drying the cellulose insulation of a
transformer at the stage of its manufacture. The research results showed the possibility of using
methanol, even with a significant concentration of water exceeding 1%, for the efficient drying of a
transformer’s cellulose insulation. It was also proved that it is possible to completely dry pressboard
of various thicknesses in a satisfactory period of time. Even in the case of thick pressboard (3.384 mm),
drying time did not exceeded 6 h.
Based on the obtained research results, the concept of a device and procedure for drying
the cellulose insulation of a transformer by means of methanol is presented. The advantages of
the proposed method include:








Maintaining the initial level of degree of cellulose polymerization after the drying process
The possibility of complete and uniform drying of the whole of cellulose insulation
Washing out post-production impurities from the insulation system
No need to apply a vacuum
Simplicity of the drying process
Short drying time

It should be noted that even methanol with a moisture of 1% can still be successfully used for the
effective drying of cellulose. Nevertheless, the repeated use of methanol in the drying process will
cause an increase in the water concentration to a level at which it will be necessary to dry it. Inexpensive
methanol treatment is a prerequisite for the cost-effectiveness of the cellulose insulation drying method.
This issue is currently the subject of further research.
Funding: This research received no external funding.
Conflicts of Interest: The author declares no conflict of interest.

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